unreinforced SOG with no sawcuts

[PG35]

I am working on a project that has a 4" Slab-on-grade with embedded in-floor heating tubes. The slab is 4000 psi concrete with fibre (the type that only reduces plastic shrinkage cracking). The tubes floated to the top of the slab (near the top of the slab) because the contractor was careless. The architect wants to eliminate the sawcuts for the slab to avoid cutting the in-floor heating tubes. They don't want to replace the sla because of the project schedule. What are the potential consequences of not providing sawcuts. I realize that the cracking will not be controlled and would occur at random. Anything else we should be concerned with?

Any input would be appreciated!

 

[SlideRuleEra]

Cracking means the concrete is moving. Depending on the type of tubing and firmly it is bonded to the concrete, you may get leakage from broken tubes.

Even if the tubes don't leak immediately but the randomly broken concrete moves because of loads applied to it, leaks could start later.

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[MarcbSE]

In this case I would probably only be concerned with uncontrolled cracking as it is only a slab on grade. If the floor finish will be flexible surfaces such as carpet then the cracking really isn't a problem. However, if the architect plans on using brittle finishes such as ceramic tile then the cracking could be a huge problem.

I would also want to make sure the current conditions are acceptable to the designer of the floor heating system. I've used these a few times in the past and if memory serves they depend on having a certain amount of concrete above the heating tubes to radiate the heat. If the tubes are at the surface it could potentially create localized hot spots that might be a problem.

 

[miecz]

As heating tubes, they will probably expand and contract radially. If the tubes are close enough to the surface, the slab may spall off over the tubes. Assuming the tubes run in one direction, I would expect the tubes to act like reinforcing, distributing the shrinkage to many small crack. In the other direction, I would expect the slab to crack at the weakest point, right over a tube, exacerbating the spalling potential.

 

[spats]

Is this a freezer facility? In that case, the heating tubes are in the sub-slab, to keep the subgrade from freezing. The sub-slab is below the underslab insulation and the wearing slab. If it's a sub-slab, I wouldn't worry about it.

I've never seen heating tubes placed in the wearing slab... seems like they would unduly weaken the slab.

 

[JAE]

SlideRuleEra - whether there are controlled, straight line joints with the resulting straight cracks, or random cracks in the slab, the resulting localized tension forces on the embedded tubes would be about the same don't you think?

 

[hokie66]

Just my opinion, but I would think the tension force on the tubes would be less with random cracks than would be the case when a tube crosses a joint. I would be most concerned with the spalling issue which miecz raised.

 

[JAE]

Well, my point is that random cracks tend to occur about the same spacing as the sawed cracks (at least that is the concept of sawed crack spacings) so the net localized separation at a random crak would be the same as that of the sawed joint/with crack.

In others words, the tube, directly at a crack, doesn't know or care whether the crack was initiated by a sawed joint or simply occurred at random. Either way the tube is pulled "x" distance apart.

 

[hokie66]

PG35 would have to answer for his project, but often saw joint spacings get specified at larger spacings than the normal crack pattern. With no joints, I expect cracks at 3 to 4.5 metre centres, while joints are often placed 6 metres apart.

 

[miecz]

The tubes will either bond or not bond to the concrete. If they bond like reinforcing, then they will try to act like reinforcing and distribute the concrete shrinkage to smaller cracks at greater frequency. If the tubes slip within the concrete, then crack spacing will be as for an unreinforced slab, but no tension will be transferred to the tubes, due to the slippage.

 

[SlideRuleEra]

JAE - If the concrete is properly mixed and placed, I agree that the effect on the tubing would probably be no worse than sawed joints.

I may be reading too much "between the lines" of the original post... but in my experience, when anything literally "floats" to to the surface of fresh concrete it is a sure indication that the concrete was so severely watered down during placement (probably by the "careless" contractor). If that is the case, then the size of the cracks and the pattern could really be random and excessive.

Also, with the tubes at the surface, there are likely planes of weakness in the slab that are dependent on the tube layout pattern. IMHO, how that slab will "behave" over time will be very unpredictable.

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[flamby]

If you have a ductile pipe (like copper) you are a bit luckier. If its like cast iron, it is doomed to crack and leak, if it is not stronger than I am imagining at the moment.

Ciao.

 

[DaveAtkins]

Aren't the heating tubes typically a flexible plastic? If so, they will not "act like reinforcing" or be affected by concrete shrinkage.

DaveAtkins

 

[csd72]

The slab will crack along the lines of the heating tubes as these will act as crack inducers.

I have seen a proprietary system designed on this exact basis.

 

[jheidt2543]

Just wondering:

Shouldn't systems like this be designed to have the fluid tubes BELOW the concrete slab? That would eliminate the problem raised by csd72. I seem to remember ice rinks, designed with the coolant tubes laid on a 4" layer of rigid insulation on the subgrade, then a sand bed, then the concrete poured.

Also, the selection of the tube material is really important. I remember an ice hockey rink built this way, with volunteer help that used black iron pipe. The leaks started at year two and eventually got so bad the floor had to be torn up to replace the piping.

 

[49merc]

If these tubes are for hydronic heating of the floor slab, you will end up with very uneven heat distribution in the slab. Whoever designed the heat system should be able to tell you the embedment needed for the system to perform according to their calculations. The performance of the system is also dependant on type of floor covering, if any, water temp, etc. The tubing is generally anchored prior to the pour by various means. Someone severely screwed up. Depending on the application here, you may be able to pour another floor on top if it won't restrict any needed headroom, or screw up the transition into other spaces. If the floor was poured too wet, it may not perform for the intended use anyway, and may need to be removed and poured again. You may be able to core a piece between the tubing to have it tested.